The latent heat absorption property of material phase change has been used as a means for absorbing heat influx and maintaining the temperature of objects in close contact or local proximity within a desired range. The phase change of water, due to the relatively large latent heat of fusion of the material, provides an excellent means of maintaining temperatures near 0° Celsius. As the presence of liquid water produced by the phase change can be inappropriate for many applications, enclosing both the solid and liquid phase in a sealed container provides a simple means of preventing water damage. To enhance container security, the container may be constructed from robust materials; however, due to the approximate ten percent volume expansion of water upon solidification, containers need to be constructed from flexible materials that do not rupture or fracture under the high expansion pressure.
Materials such as plastics and rubbers are used to construct such expandable containers. To reduce the container thickness while managing the risk of a rupture spill, water is often absorbed into materials such as gels, foams, and fibers, and enclosed in sealed bags or containers. Such options are frequently applied where costs and weight reduction is desired, as in shipping and transport applications.
Unfortunately, however, such containment options are also typically associated with insulating properties that restrict the flow of thermal energy to the phase change medium. Plastic and rubber container materials have a low thermal conductivity and effectively insulate the phase change material contained therein. Absorptive materials also present an insulating feature in that the materials will thaw from the outside inward as heat is absorbed. The thawed material restricts the transfer of thermal energy to the solid remaining core, thereby imposing an increasingly thicker insulation barrier as the phase change progresses. Placing an insulation barrier between the solid phase of the phase change material and the object that is to be thermally regulated increases the dynamic effective temperature of the material or device. As the effective insulation barrier thickens, the temperature of the object will rise and may exceed the desired temperature range.
While a variety of devices and materials require cooling or maintenance at a cool (below ambient room temperature, i.e., around 0° Celsius), biological materials (organs, tissues, cells, cellular components, proteins, nucleic acids, and the like) are frequently maintained at cool temperatures, because the natural breakdown of biological materials can be significantly delayed by refrigeration. While many types of biological specimens can be preserved for an even greater duration by freezing the material, freezing is inappropriate for many biological samples. Tissue structures can be disrupted by ice crystal formation, thereby desegregating labile and degradative components. For example, specimen solutions can be damaged by ice crystal formation, as well, and concentrated solutes may impose conditions of pH and salt tonicity that alter molecular structures. As a result it is desirable to maintain biological specimens at a temperature that is above 0° Celsius and below 4° Celsius. Although this temperature range can be easily achieved by placing specimens into crushed ice or into ice water, safety, energy management, ergonomic, clinical protocol, space restriction, and sterility concerns have created a significant need for portable cooling solutions without exposed ice. Aqueous gels, contained water, and absorbed water-based phase change solutions currently fulfill the need for thermal sinks on which portable passive cooling solutions can operate. However, due to the construction of the thermal sink units, a steady temperature near the phase-change temperature of the thermal sink medium is difficult to maintain.
Numerous substances with temperature sensitivities, including biological samples, chemicals, and drugs are subject to degradation when shipped by common methods using gel packs and insulated containers. Unless the payload of the package is in intimate contact with the phase change medium, thermal gradients inside the package can result in significant elevations in temperature in addition to temperature fluctuations as package contents rearrange during shipment. As the gel packs thaw during normal use, the added thawed material on the gel pack boundary adds more separation from the frozen core, further increasing the temperature differential thereby.
Therefore, there is a need for a phase-change container that will isolate the phase change material, allow for expansion upon solidification of the contained material, provide a thermally conductive interface with the object to be thermally regulated, and ensure close proximity of the solid phase of the phase change material to the thermally conductive barrier, thereby cooling an object and/or maintaining the cooled object in a narrow temperature range close to phase change media transition temperature. The present invention meets these needs.